As technologies further advance, wireless power transfer has emerged as an efficient and convenient mechanism for powering or charging battery based mobile devices such as mobile phones, tablet PCs, digital cameras, MP3 players and/or the like. A wireless power transfer system typically comprises a primary side transmitter and a secondary side receiver. The primary side transmitter is magnetically coupled to the secondary side receiver through a magnetic coupling. The magnetic coupling may be implemented as a loosely coupled transformer having a primary side coil formed in the primary side transmitter and a secondary side coil formed in the secondary side receiver.
The primary side transmitter may comprise a power conversion unit such as a primary side of a power converter. The power conversion unit is coupled to a power source and is capable of converting electrical power to wireless power signals. The secondary side receiver is able to receive the wireless power signals through the loosely coupled transformer and convert the received wireless power signals to electrical power suitable for a load.
In a battery based system such as a mobile phone, the top surface of a transmitter may be implemented as a charging pad. The transmitter coil can be placed underneath the charging pad. The mobile phone may receive power from the charging pad when it is placed near or on the charging pad. In particular, the receiver coil in the mobile phone picks up the power transferred from the transmitter coil through magnetic coupling between these two coils. The distance between the transmitter coil and the receiver coil is in a range from about 1 mm to about 100 mm. A high frequency current in the transmitter is desired in order to transfer power efficiently over a long distance. However, the achievable frequency in a wireless power transfer system is usually limited by the switching losses of the power switches of the wireless power transfer system.
As power consumption has become more important, there may be a need for high power density and high efficiency wireless power transfer systems. Resonant converter based wireless power transfer systems have become the preferred choice for achieving high performance (e.g., lower power losses) because resonant converters are capable of reducing switching losses of power switches through zero voltage switching and/or zero current switching. However, as the frequency of the wireless power transfer system goes higher, the EMI compliance has become a significant issue, which presents challenges to the system design of the wireless power transfer system.